The present invention relates to blind fasteners in general and more in particular to a blind fastener having an expandable sleeve for bearing on the backside of a joint and applying a controlled low unit load on the backside.
Blind fasteners fasten two or more sheets together from a single side. The fastener is inserted from a setting side of the sheets into aligned holes in the sheets. An element of the fastener on the blind or inaccessible side of the sheets is expanded and brought into bearing against the blind side. A head on the setting side coacts with the expanded element to clamp up the sheets by applying a compressive load on them.
One form of blind fastener has a hole filling sleeve that has a manufactured head and a nose for extending from the backside sheet. An expandable portion of the sleeve is a portion of the nose. A pin drawn axially by a large force compressively fails the expandable portion and bulbs it by reacting it against the backside sheet. The expandable portion can be made to separate from the sleeve proper by providing a fracture zone between them. Further, the expandable portion can be externally tapered so that the thinnest cross section is against the backside sheet at the outset of bulbing. The pin has a breakneck that fails upon reaching a predetermined load during setting and at failure of the breakneck the fastener is set. One such fastener is described in U.S. Pat. No. 3,277,771 to Reynolds. A problem with this fastener is that it is possible to form a bulb out of contact with the backside sheet without being aware of it. Furthermore, as compared with a threaded fastener, the setting load is high because the mechanical advantage of the threads is not present.
The external taper of the sleeve in Reynolds, also, does not develop the external diameter and bearing area of an externally cylindrical sleeve because the diameter is necessarily less at the critical zone of failure next to the backside sheet. This type of problem was recognized in U.S. Pat. No. 3,253,495 to Orloff who proposes to band anneal an expandable section of a hole filling sleeve so that there is a strength gradient with strength increasing away from the backside sheet and head of the sleeve. This promotes bulbing at the backside sheet. Orloff also suggests that a weak zone can be presented to the backside sheet by varying the cross-sectional area of the sleeve to get a strength gradient. U.S. Pat. No. 2,562,019 to Colley shows a rivet set with a mandrel and having an internally tapered bore with a shank thickness progressively greater away from the work to promote bulbing on the backside of the sheet. Among other things, the Colley construction is not, however, otherwise sensitive to setting load and runs the taper into the zone within a sheet, which reduces shear strength.
Another type of blind fastener has a nut for receiving a setting screw or core bolt and a separate sleeve as the expandable element. The sleeve is expanded on the blind side of the joint by tightening the core bolt and nut. The nut has a manufactured head for bearing on the setting side of a joint. This head may have wrenching flats for purchase of a wrenching tool and prevention of nut rotation during setting. The manufactured head may be protruding or flush. The nut has a shank with a diameter for substantially complete occupancy of the aligned holes in the sheets. A blind side end of the nut, the end opposite the manufactured head, is a nose which externally tapers to provide an expansion "ramp" surface over which the sleeve expands. The sleeve is cylindrical and has an external diameter no greater than the diameter of the nut so that the sleeve passes through the aligned holes in the sheets and an internal diameter for receipt of the core bolt. A core bolt head has a diameter no greater than the diameter of the sleeve for passage to the blind side and a radial shoulder for bearing on the end of the sleeve. The core bolt has means on the setting side to tighten it in the female threads of the nut, which means may be opposed wrenching flats. As the core bolt is threaded relative to the nut, the screw head bears on the sleeve and the sleeve is forced over the nose of the nut and expanded against the blind side of a joint. Load determining means such as a breakneck in the screw can fail to stop tightening and determine the amount of clamp-up force. This type of fastener is described in U.S. Pat. No. 3,596,948 to Thomas F. Spoehr and U.S. Pat. No. 3,443,474 to Blakeley and Bergere.
The clamp-up force applied to the joint axially of the fastener between the expanded sleeve and the manufactured head of the nut distributes to the blind side of the joint through the area of the joint-sleeve bearing. With frangible materials such as graphite-epoxy composite structures, the unit loading applied by this type of fastener can fail the material in compression.
A third type of blind fastener has a threadless sleeve in place of the nut of the fastener just described. The sleeve has a manufactured head for bearing on the setting side of a joint. A threaded nut has a thin-walled section that collapses against the backside sheet with setting. The thin-walled section bears on a shoulder of the nut. A threaded mandrel engages the threads of the nut portion and draws the nut axially without turning in the nut portion threads. After setting, the setting mandrel is withdrawn and a core bolt installed to complete a joint. This type of fastener is described in U.S. Pat. No. 3,369,442 to Darby et al.
The type of fastener just described can have a tendency to "bulb in the air." The blind side bulb formed upon pulling on a setting mandrel forms but does not always bear against the backside sheet. A condition where this can occur is when the nut portion is loaded eccentrically with respect to the longitudinal axis of the sleeve. Also, setting requires the removal of the setting mandrel and installation of the core bolt, a step not required in the first fastener described.
Accordingly, there is a need for a blind fastener that applies a low unit load to the backside sheet of a joint, that does not fail that sheet, that provides adequate clamp-up force on the joint, that reliably forms a backside bulb which clamps up the sheets, and that forms a joint without a setting mandrel.